Chromatography is a set of techniques for separating a mixture into its constituents. For instance, in a liquid chromatography application, a pump takes in and delivers a mixture of liquid solvents to an autosampler, where a sample awaits the mixture's arrival. In an isocratic chromatography application, the composition of the liquid solvents remains unchanged, whereas in a gradient chromatography application, the solvent composition varies over time. The mobile phase, comprised of a sample dissolved in the solvent stream, passes to a column of particulate matter, referred to as the stationary phase. By passing the mixture through the column, the various components in the sample separate from each other at different rates and thus elute from the column at different times. A detector receives the elution from the column and produces an output from which the identity and quantity of the analytes may be determined.
Historically, the pump's delivery of the solvent stream to the autosampler did not start until after the autosampler completed its pre-injection operations and was ready to initiate the actual injection event (i.e., a valve transition to introduce the sample to the stream). Thus, the process was sequential: first, start and finish the autosampler's pre-injection operations and, then, start and finish the solvent stream delivery.
Before the solvent stream reaches the injector, however, and thus before injection can take place, a pre-injector dwell volume (also called the delay volume) must first be delivered. In general, the pre-injector dwell volume is the volume of liquid from the point where the solvent mixture forms to the injector valve. Depending upon various factors, such as the use of a mixer, pre-injector dwell volumes can be relatively large (in hundreds of microliters), and, therefore, introduce a sizeable delay to the start of each injection. Historically, however, this delay was insignificant compared to the lengthy chromatographic run times that took tens of minutes and even hours.
This delay becomes problematic, though, when run times are in terms of just a few minutes. For example, if the pre-injector dwell volume is approximately 400 μL, and the pump produces a flow rate of 350 μL per minute, then the time for the solvent stream to reach the injector valve is over a minute. Thus, the dead time introduced by the pre-injector dwell volume can be a significant percentage of a run, particularly when run times are only minutes in length.